Well, here's another suggestion,
why not setting, for each intermodal station, a matrix where it can be set what the maximum percentage of a specific cargo type (e.g. pasengers) coming from one trasportation system (e.g. road vehicles) can be loaded by another trasportation system (e.g. airplanes). The percentage should refer to the loading camapcity of each vehicle or could just be a yes/no (taht is either 0% or 100%) for simplicity.
So, for example, if you have an airport/bus station combo buses could not load more than a given percentage (or even any) passengers that were transfered to the stationby other buses, while they may load passengers transfered to the station by airplanes, and viceversa. I think also that Helicopter should be considered separately from airplanes (or it should be possible to do so).
Herunder you'll find a broader rationale for my suggestion.
I read on
OPENTTD's wiki about setting up an intermodal feeder system, for example:
1. load a bus with people
2. take them to a combo airport/bus station and tranfer them, to prevent the bus form reloading the passengers it just loaded set it to "unload" or "transfer and do not load" (they look liek working the same to me, or am I wrong?)
3. load them on an airplane
4. take the airplane to another airport and either "deliver" them (if the airport accepts passengers) or, again, unload (or transfer and do not load) them and take them to final destination (i.e. any kind of station accepting passengers) through another vehicle (if the airport does not accept passengers, or if you're willing to make a more complex intermodal system).
One problem arises: the first bus must always travel empty from the airport to the bus station, and the airplane (in case you're making it transfer and not deliver passengers) must also travel empty from the 2nd airport (point 4) to the 1st one. The only way to make an efficient 2 way feeder system (I think) would be to have separate airports and vehicles for either transportation direction, or at least separate airports in each town for either trasportation direction.
Obviously a multipoint feeder system with more than two transportation systems (trains, buses, airplanes, ships) merging at the same hubs would pose even bigger issues, giving a solution to which would need even far more unefficient (and unrealistic) solutions.
The cargo source/direction and the option for not letting cargo go back from where it comes would not completely solve the issue (though it would give many other possibilities) while requiring every station to heve a list of every cargo piece it receives along with the source it comes from and, possibly, the destination it goes to (or wants to go to?, I don't know much about the patches that should introduce cargo source/destination), which, in a complex game would be a pretty huge amount of data to handle.
This could anyway be combined with the station based transfer percentile system I suggest.
Wth the station based transfer percentile system multimodal hubbing would become far more efficient and also gamplay would be more challenging (e.g. I can set buses not to load passengers transfered to the airport/bu station combo by other buses, but I then have to provide enough airplens transporting the pasengers away, otherwise I won't get any money for these passengers since tranfer fees are cashed only when the passengers are finally delivered, but if I have too many airplanes then they will mostly travel empty and make poor revenues, while still costing for buying and performance)
Furthermore i read
here about a patch allowing for percentile loading of trains. If thsi coud be made for unloding, tranfering or a combination of them and this was combined with my suggestion the possibilities would be terrific, e.g. an airplane landing to a combo airport/bus station hub accepting passengers could "trasnfer" only part of its cargo while "finally delivering" the rest....
Sorry for the long monologue
darkpella